sidechain

Here, R is the sidechain, sometimes called the R-group. The remaining atoms belong to the backbone, a central polymer which connects the protein together. For the twenty natural amino acids, there are twenty different sidechains. The simplest sidechain is a hydrogen, resulting in the amino acid glycine. The biggest sidechain is 3-methylindole, which when attached to a backbone unit, gives tryptophan. The different physical and chemical characteristics of sidechains determine how a protein assembles, and gives it a basic toolset to build environments which can carry out complex chemistry, or make very specific interactions.

In audio, sidechain refers to a signal which has branched
away from the audio path of a device, and will not appear on the
output. It usually controls a processor which does affect
the audio path directly.

For example, a compressor or gate takes a sidechain, and determines its level,
using this to control a VCA which directly affects the output.

Some devices bring the sidechain to the panel, to allow an external signal to control the processor, or to have a processed sidechain sent to another device. Examples of such devices are DBX 1066 and BlueMAXstereocompressor, which allow a signal to be compressed according to the output of another signal.